1. Field of the Invention
The invention relates to polymer mixtures, particularly mixtures comprised of an aromatic polycarbonate and a copolymer of methyl methacrylate, which are highly transparent.
2. Discussion of the Background
It is known to mix aromatic polycarbonates, particularly polycarbonates of bisphenol A, with other polymers resulting in compositions which may be multiphasic polymer mixtures, i.e. showing incompatibility (so-called "polymer blends"), or which may be homogenous, i.e. showing compatibility (often called "polymer alloys").
Mixtures of a polymethyl methacrylate molding compound which is a transparent plastic of glass-like clarity, with bisphenol A polycarbonate, which is also a transparent plastic, are not homogenous, transparent "alloys", but rather are mixtures with a pearl-like luster (as such are disclosed in, e.g., Jap. Pat. No. 72-16063).
According to Ger. OS No. 22 64 268, an improved polycarbonate molding compound is obtained if there is added to the polycarbonate an acrylic polymer with low molecular weight, comprising a copolymer of methyl methacrylate in the amount of 90-75 wt. % and an alkyl acrylate in the amount of 10-25 wt. %, which is of the formula ##STR1## where X represents H or CH.sub.3, and R represents an organic group with 4-12 carbon atoms. The copolymer may be added in the amount of 0.01 wt. % to about 50 wt. % based on the weight of the polycarbonate (i.e. of the mixture) without detrimentally affecting the transparence. The viscosity reduction effect increases with increasing amounts of copolymer.
Such copolymers act as polymeric plasticizers. Their molecular weight must be less than 15,000 in order to achieve the described compatibility, according to the present authors' studies with corresponding copolymers.
However, there is a problem in using such polymers to produce polymer "alloys" having technically useful properties in the region of high polymethacrylate content, the problem being namely the known inferiority of mechanical properties in the molecular weight region below 100,000, particularly below 50,000 (see Vieweg and Esser, "Kunststoff-Handbuch", Vol. IX, "Polymethacrylates", pp. 112 ff.).
Ger. Pat. App. No. P 35 18 538.4 describes compatible polymer mixtures comprised of a polycarbonate such as bisphenol A polycarbonate and a copolymer comprised of (a) monomeric esters of acrylic and/or methacrylic acid; (b) alcohols with 1-10 carbon atoms; and (c) a UV-absorbing monomer of formula ##STR2## where R.sup.1 is hydrogen or a methyl group, and Y is oxygen or a --NR.sup.2 --group, where R.sup.2 is hydrogen or an alkyl group, and Z is a UV-absorbing group, namely a 2-hydroxyphenylbenzotriazol group, a 2-hydroxybenzo- or acetophenon group, or a .alpha.-cyano-.beta.,62 -diphenyl group.
Such thermoplastically processible polymer mixtures of polycarbonate and copolymers of methyl methacrylate and monomers with good UV absorption are useful as light-protection agents for improving light stability of, e.g. plastics, particularly when used as coatings for the plastics. However, they remain in the specialty area, and are prohibitively costly for broadly applicable thermoplastically processible plastics because of the high cost of the UV absorbers which they incorporate.
There remains a need for compatible mixtures with technically important properties, comprised of a thermoplastic polycarbonate and thermoplastically processible polymers with high proportions of methyl methacrylate and with molecular weights above 30,000, for use in a broad range of applications of the sort typically found for molding compositions.